NFS: Clean up the auth flavour array mess

What is the point of having a 'auth_flavor_len' field, if it is
always set to 1, and can't be used to determine if the user has
selected an auth flavour?
This cleanup goes back to using auth_flavor_len for its original
intended purpose, and gets rid of the ad-hoc replacements.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>